Okay, nobody gets banned yet.
Just sayin. You can't ban someone from looking broken, or preforming well. Only if it essentially can kill things without counters, it shouldn't be banned.
So whens the tournament? It hasen't been confirmed yet right?
Rhydon doesn't have counters. There are things called "checks".
Otherwise Salamence and Garchomp wouldn't be banned. There are Ice shards from Weavile and Mamoswine. And certain Hidden Power Ices. They are banned because they have a STAB Attack that only one type resists, and they have super effective moves (Another STAB in Garchomp's Cade) against that type (steel). As for Rhydon; Well, that attack is OMG in MC, even higher than Magneton and Kadabra's Special Attack. It does have a slow speed and glaring weaknesses, but a true counter for it is hard if it switches in on the right pokemon. For example, it switches in on a Chansey, and proceeds with an attack, or subsitute (rock polish is not recommended since Chansey may status it). Chances are Rhydon will outspeed Chansey since Chansey doesn't run Speed Evs, and: (Note: Stone Edge is generally not used due to it's bad coverage for it's counters; e.g. Marshtomp and Grotle. Megahorn is quite useful to deal with Grotle, but Rhydon doesn't learn any Grass moves to counter Marshtomp)
EDIT: I forgot that Grotle wasn't part Ground... But Megahorn is still the best switch in move, and does lethal damage to everything except for Dusknoir and Haunter (I think...)
END EDIT
Here is the prediction list.
1. Rhydon uses Attack. Chansey dies.
2. Rhydon uses Attack. Chansey switches. Incomer suffers major damage.
3. Rhydon uses Substitute. Chasey Statuses. Chansey wastes turn.
4. Rhydon uses Substitute. Chansey Switches. Rhydin gets free hit on incomer.
Basically it's always win for Rhydon in this case. Note that this is just an example case, which is only hypothetical, although Chansey is quite common in MC...
Let's see how good Grotle and Marshtomp are at counter it now. (OK, there are other Water and Grass types out there, but they are just checks for Rhydon. They either are Outsped or too weak against other pokemon. In the former case, Rhydon kills it. In the latter case, Rhydon switches out. I'll list the pokemon just in case you are wondering):
Piloswine:
STAB Ice Shard, but you just swich Monferno or something else in and Pwn it.
Seadra:
Without Swift Swim it really is downgraded. Just get Chansey, Electabuzz or a grass in. Also can't switch in on Rhydon.
Sealeo:
Too slow to be useful. Rhydon is probably running Plus Speed Nature anyways.
Ivysaur:
One of the better checks. But can't switch in onto Rhydon.
Bayleef:
Ivysaur is better. Has Megahorn Weakness.
Grovyle:
This guy is actually quite interesting. But it's too fragile for Rhydon (switch ins).
Wartortle:
Useless against all others. Only Rapid spinner makes it valuable, but Rhydon switches out.
Crocanow:
No reason to run surf. Croconaw without Life Orb can't OHKO it if it runs Max Def. If Rhydon doesn't run Max Def just switch out.
Prinplup:
Uselss against everything else. The other water starters are suprerior (like how Quilava and Charmeleon are forgotton, Combusken is Ok, but everyone uses Monferno (or Magmar))
Kadabra:
It probably won't get to switch into Rhydon anyways. If it does somehow, switch out to something like Chansey. Or Nuzleaf (Just Kidding).
Haunter:
What Kadabra said. Other than that, Normals wall it pretty bad since it doesn't have Focus Blast so switch in Porygon 2 or Vigoroth or Loudred (lol)
It also has an easier time switching in due to Earthquake being useless and Megahorn being near useless.
Other Pokemon like Weepinbell or Marill are generally too weak to do anything else other than hitting Rhydon with a super effective attack.
Now let's get on to Marshtomp and Grotle;
Marshtomp:
The main thing that makes Marshtomp quite good against Rhydon is not only because of the Water damage it deals, but also it's resistance to Stone Edge.
a 252 EV Waterfall vs. 252 EV Defense Rhydon = 78.6% - 93.4%, which means it won't KO. On the other hand, Rhydon's Mega horn does (0 Ev for Marshtomp, 252 EV for Rhydon) is 63.3% - 73.7%. Earthquake does 78.3% - 92.2%. If Rhydon gets an attack on Marshtomp when it switches in, or gets Substitue up, Marshtomp cannot counter it that badly. Marshtomp doesn't really have a reason to run Surf anyways. And if it does, it's purpose would probably just be to get rid of Rhydon.
Grotle:
Now this guy is weak to Megahorn and is slower than Rhydon, but if Rhydon doesn't run Megahorn Rhydon will find a hard time kiling it Grotle's Seed Bomb does 79.8% - 95.7%, which is quite good, since if Rhydon doesn't run max defense, or has prior SR damage, it might be OHKOed. Megahorn will OHKO Grotle for sure, but Earthquake only does 32% - 38.1%, and Stone Edge does 64.3% - 76.3%. Still, if Rhydon does Prior damage on the switch, Grotle dies.
Yeah, I didn't mean to write this long >.>
By any chance, Rhydon is a very strong Pokemon in MC.
I'm not saying it should be banned immediately. Lets just see how it goes.